There have heretofore been two methods, that is, the addition method and the removal method, for correcting the imbalance of an armature of an electric motor, as discussed in a publication titled "Method of Balancing Small-Sized Motor Rotor" by Shigesawa, Kawamori and Nakayama, on pp. 173 to 177 in Shimazu Hyoron Vol. 32, No. 2 (1975). In correction by the addition method, material such as is resin is added to the outer pheripheral portion of the armature. However, it is difficult to make adjustment by adding a minute amount of the resin. On the other hand, in correction by the removal method, it is possible to effect a minute adjustment relatively easily. However, when the outer periphery of the armature core is partially eliminated, the air gap defined between the same and the field system is theoretically changed. In consequence, in a permanent magnet type DC motor in particular, the second method of correction definitely involves the disadvantge that a magnetic vibration is easily generated.